If you decided to go custom aftermarket in the M, this would be a great high quality and very clean sound.
OEM Interface: Audison Bit Ten D
Amp: Audison LRx 6.9
Speakers Front: Hertz HSK 165
Speakers Rear: Hertz HCX 165 (if hsk 165 don't fit in rear)
Sub: Hertz HX 250 D (custom box)
I put exactly this in an r32 but thinking about putting it in my M. It truly had an incredibly clean sound.